1. Field of the Invention
The invention relates generally to data storage techniques employed in conjunction with computing systems, and more specifically to error-tolerant techniques for recovering byte synchronization information from data storage media.
2. Description of the Prior Art
Byte synchronization recovery schemes are often employed in conjunction with data storage units. Many data storage units, such as magnetic disks, store data in concentric circles called tracks. These tracks are divided into blocks called sectors. A sector normally includes a data field, an identification (ID) field, a phase-lock oscillator (PLO) field, and a byte synchronization ("SYNC") field. A sector may further include a cyclic redundancy check (CRC) field, an error correction code (ECC) field, one or more padding (PAD) fields to fill out a sector, and one or more inter-sector gap fields.
Within a sector, synchronization of the data and ID fields is achieved by detecting a particular bit pattern in the synchronization (SYNC) field. The SYNC field enables a determination of the locations of the boundaries between adjacent bytes on the data storage unit. This process may be referred to as "byte synchronization." The SYNC field also enables a determination of the position of a particular byte with respect to the remaining data on the data storage unit. In the case of a magnetic disk, the SYNC field is typically placed immediately after each PLO field and right before the corresponding ID or data field at the time of formatting the disk.
One purpose of the bit pattern in the SYNC field is to provide proper synchronization for the beginning byte of the ID or data fields at the time of a data Read operation. The SYNC field is generally one byte in length, and the field contains a predetermined pattern of bits, referred to as the byte synchronization pattern. The search for the byte synchronization pattern is done over a certain period of time, which is called the "SYNC search window. If a particular byte SYNC pattern is not detected within that window, the information stored in the corresponding sector cannot be retrieved. If there is an error in the SYNC field, either a loss of data or a false detection of synchronization will result. In either case, access to the information stored in the corresponding sector will not be provided, and the data is effectively lost.
Errors in the SYNC field may be characterized as random bit errors. However, due to the fact that the information stored in the SYNC field is processed by a modulation decoder, a random bit error may be transformed into a data burst error affecting a plurality of data bits. The process of byte synchronization occurs after the data bits from the SYNC search window are passed through the modulation decoder. Therefore, data burst errors may prevent synchronization or may cause false synchronization to occur. Commonly-utilized modulation codes have been developed to minimize the length of such data burst errors. For example, the well-known run-length-limited (1,7) and (2,7) codes place inherent limits upon the error propagation properties of the modulation decoder by ensuring that a single-bit error cannot generate a data burst error more than six bits in length.
In the context of a data storage unit, data burst errors affecting the SYNC field are particularly troublesome. For example, the organization of a typical sector on a disk requires that the byte synchronization be recovered two times per sector. The SYNC search window normally begins near the middle of the PLO field. In case of an error in the SYNC field, presently-existing synchronization hardware will simply fail or falsely detect the synchronization. In other words, the existing scheme does not provide any fault tolerance, and thus is very vulnerable to bit errors. No conventional error correction code can be employed to recover from an error in the SYNC field, since any ECC requires bit/byte synchronization.
Hence, a fault-tolerant synchronization detection system is desirable to avoid the disastrous situation of data loss from errors in the SYNC fields, and to achieve high system reliability and performance. The purpose of fault-tolerant synchronization recovery schemes is to enable synchronization even if there is an error in the SYNC field. What is needed is a fault-tolerant synchronization detection scheme which will recover synchronization correctly from short burst errors in a SYNC field. Such a scheme may be employed in conjunction with improved synchronization bit patterns to improve error tolerance.
The present invention provides such a scheme, as well as a method for determining such improved synchronization bit patterns.